This invention relates to the dissolution and recovery of copper from copper and iron-bearing ores or concentrates particularly sulfide ores or concentrates. The invention allows at a low cost the substantially pollution-free production of copper metal at substantially atmospheric pressure in a single step.
A well known prior art treatment for copper-bearing ores, particularly sulfides, is the pyrometallurgical treatment of the ores. The pyrometallurgical treatment of sulfide ores is expensive, pollutes and requires the disposal of large quantities of sulfur dioxide or by-product sulphuric acid. To overcome the disadvantages of prior art pyrometallurgical processes, particularly pollution, a number of hydrometallurgical processes have been developed particularly aimed at the recovery of copper from ores containing chalcopyrite, malachite or azurite.
Chalcopyrtie (Cu Fe S.sub.2) is one of the most common copper materials and also one of the more oxidation-resistant minerals, being reported as more noble or oxidation-resistant than pentlandite, cobaltite sphalerite, galena, chaleocite and pyrrhotite (least resistant) and less noble than pyrite and molybdenite.
Chalcopyrite may be oxidized according to the equations: EQU Cu Fe S.sub.2 .fwdarw. Cu.sup.+ + Fe.sup.++ + 2S + 3e (1)
or EQU Cu Fe S.sub.2 .fwdarw. Cu.sup.++ + Fe.sup.+++ + 2S + 5e (2)
depending on the degree to which the oxidation is taken. Due to the instability of sulfur in alkaline oxidizing solutions, such reactions must be carried out at a pH of less than about 7.
This may be accomplished in autoclaves using high pressure air or oxygen in a similar manner to that described in Canadian Pat. No. 618,623 (for zinc ores) (Sherritt Gordon Mines Ltd.), however, the plant and operating costs are extremely high.
Australian patent application 52833/73 (The Anaconda Co.) describes a process for the treatment with oxygen of copper sulfides in an ammoniacal solution in low-pressure autoclaves. The process is expensive, requires large amounts of ammonia, oxygen and a ready market for ammonium sulfate which is produced in quantities of five to ten times those of copper.
U.S. Pat. No. 3,673,061 (Cyprus Metallurgical Processes Corporation) describes the oxidation of copper sulfides at the anode of an electrochemical cell. The presence of iron in the ore results in a low anode current efficiency due to the power consumed by iron oxidation reactions, as is apparent from equations (1) and (2) above. From equation (1) it can be seen that a three electron exchange is necessary for the dissolution of one atom of copper while deposition would only require a one electron exchange thereby resulting in an anodic dissolution current efficiency of only 33% with respect to copper. To overcome this inefficiency, it is necessary to produce electrolytic iron in amounts similar to those of copper and thereby requiring an equivalent market for the iron product which is produced by a very expensive method. In addition, further care must be taken to avoid decreases in current efficiency due to further oxidation of elemental sulfur to sulfate. The process requires high anode current densities which result in increased anode wear and requires care in the electrolytic recovery of copper due to the high iron content of the electrolyte.
Australian patent application No. 54656/73 (Cyprus Metallurgical Processes Corporation) is very similar to the above patent and suffers from the same disadvantages.
Another Australian patent application No. 56990/73 (Hazen Research Inc.) discloses a process for the leaching of sulfide ores with ferric chloride which is regenerated anodically. The process requires a continuous electrolyte flow from the catholyte to the anoltye to avoid the severe reduction in current efficiency which would result from ferric ions reaching the cathode. There is also no provision for a method of overcoming the inefficiencies due to the presence of iron sulfides in the ore.
Australian patent application No. 46913/72 (Duval Corporation and corresponding to U.S. Pat. No. 3,785,944, Atwood et al, Jan. 15, 1974) describes a relatively complex cyclic process involving the ferric chloride and cupric chloride leaching of copper sulfide ores and the regeneration of ferric chloride with oxygen. U.S. Pat. No. 3,923,616 to Atadan, Dec. 2, 1975, also describes a cupric/ferric chloride leach process. Atadan is largely concerned with at least a two-stage process, in the first stage of which cupric chloride is reduced to cuprous chloride by reacting with unreacted chalcopyrite. Atadan also describes a final stage in which the ore, that is, the reacted ore of the first stage in a multistage process or unreacted ore in a single stage process, is subjected to an oxygen-bearing gas under pressure in a hot electrolyte containing an excess of chloride ions to produce cupric chloride (the thus-formed cupric chloride being fed back to the first stage). The requirement of a final oxidation at elevated pressure is generally attributed to the formation of an inert, adherent sulfur film on the ore or concentrate surface during initial treatment of the ore or concentrate with one or more strong oxidizers such as cupric chloride or ferric chloride. In such elevated pressure systems, it is not practical to use air as the source of oxygen because, among other things, of the need to vent large quantities of gas in which the oxygen concentration has been reduced below a useful level.